Wavelength division multiplexing methods and apparatus for constructing photonic beamforming networks

ABSTRACT

Methods and apparatus for constructing phased array antenna beamforming networks are provided, that allow to scan multiple beams and select appropriate sets of delay lines simultaneously. The beamforming networks disclosed herein generate less losses than conventional ones and in some cases, do not require active switching, making them completely passive. Three main methods are comprised in the invention: (1) laser wavelength hierarchies, (2) arrangements of Wavelengths Division Multiplexing (WDM) components, (3) re-use of laser wavelengths. Multiple laser wavelengths are arranged in groups and subgroups (wavelength hierarchies) in the wavelength domain. By switching between these wavelength groupings, the arrangements of WDM components disclosed herein enable the beamforming network to direct the beam signals to the proper time delay lines, and to differentiate multiple beams. The method of laser wavelength re-use permits to significantly reduce the number of wavelengths utilized, and thus to limit them to the standard wavelengths specified by the ITU.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation application of U.S. Ser. No.10/632,354, filed on Jul. 31, 2003, (now issued U.S. Pat. No. 7,026,988)which is a divisional application of U.S. Ser. No. 10/196,480, filed onJul. 15, 2002 (now issued U.S. Pat. No. 6,844,848), which is adivisional application of U.S. Ser. No. 09/593,188, filed on Jun. 14,2000 (now issued U.S. Pat. No. 6,452,546). This application may also berelated by subject matter to U.S. Ser. No. 10/196,444, filed on Jul. 15,2002 (now issued U.S. Pat. No. 6,828,934) and U.S. Ser. No. 10/696,607,filed on Oct. 28, 2003.

FIELD OF THE INVENTION

This invention relates to the field of phased array antennas that arecontrolled by networks of optical fibers and other photonic components,such as photonic beamforming networks. More specifically, it relates tomethods for constructing low-loss, passive photonic beamformingnetworks.

BACKGROUND OF THE INVENTION

Phased array antenna systems are widely used in radar, electronicwarfare and high data-rate communications applications. They aresometimes controlled by networks of optical fibers and other photoniccomponents such as lasers, fiber splitters/combiners, andphotodetectors. These control networks mainly utilize delay linenetworks such as the ones shown in FIG. 1. There are two types of delayline networks which differ in the way time delays are implemented. Inthe network switched architecture of FIG. 1 a, of which the Rotman lensis an example, entire networks of delay lines are switched in/out by asingle switch. In the in-line switched architecture of FIG. 1 b, thereare several delay lines within each fiber as well as a switch to selectthem. If F is the number of fibers and P the number of delay states, thenetwork switched architecture requires one switch with P states, and thein-line switched architecture F switches with P states. Both require 1×Psplitters to access P delay states, and F×1 combiners to vector sum theoutputs. For both types of networks, the signal passes through one 1×Psplitter, one switch, and one F×1 combiner, so the losses are expectedto be comparable.

The number of photodetectors required in the network can be a major costdriver so it is desirable to minimize it. To achieve this, one can placea single photodetector at position A in FIGS. 1 a and 1 b, after the F×1combiner which vector sums the fiber signals. However, if all fiberscarry the same optical wavelength, as it is the case in most prior artsystems, the different signals will interfere and unwanted noise willappear on the detected carrier envelope. In order to avoid this opticalcoherence problem, photodetectors can be placed at positions B so thatphotodetection occurs prior to summation, and the optical carriers neverinteract. However, a large number of photodetectors is then required andcost is greatly increased.

To solve optical coherence problems, while still minimizing the numberof photodetectors required, this invention utilizes multiple opticalwavelengths. This reduces photodetector count from F×P to 1 in thenetwork switched case (FIG. 1 a), and from F to 1, in the in-lineswitched case (FIG. 1 b).

Furthermore, in accordance with this invention, the lossysplitters/combiners that form the actively switched prior art networksof FIGS. 1 a and 1 b, are replaced by a passive Wavelength DivisionMultiplexing (WDM) network. The 1×P splitters and F×1 combiners arereplaced with WDMs, and the functions performed by active switches arerealized by separating wavelength groups with passive WDMs. Opticallosses in a 1×N WDM are less than in a 1×N splitter or combiner for N>6,so in most practical cases losses can be substantially reduced.

Prior art photonic networks require active switching, the use of a largenumber of photodetectors, and inclusion within the network of lossysplitters and combiners. In many cases the prior art also requiresspecialized or unique optical components.

Prior photonic beamforming art such as described in U.S. Pat. No.5,861,845 (Wideband Phased Array Antennas and Methods) alludes to usingmultiple wavelengths to avoid optical coherence effects, but losses arestill high in the combiners which vector sum the optical signals. Patentapplication Ser. No. 09/383,819 (Phased Array Antenna Beamformer)describes a passive receiver network for multiple beams which employsWDMs for beam scanning and delay line selection. However, it does notaddress optical coherence problems, and uses three-dimensional fiberoptics based delay line networks (fiber Rotman lens) which are hard tofabricate. It also utilizes lossy combiners for signal summation.

The present invention addresses and solves these problems in a simple,unified manner, and can be implemented using standard ITU (InternationalTelecommunication Union) components developed commercially for fiberoptics data networks, and two-dimensional SOS (Silicon on Sapphire)fabrication techniques.

SUMMARY OF THE INVENTION

In accordance with the present invention, a number N of incoming RFwavefronts are simultaneously received by an antenna array. Laser lightis amplitude modulated to provide B=N synthesized optical beams. Thesynthesized optical beams are mixed with the incoming electricalwavefronts by optical modulation. The resultant N optical wavefronts,all traveling through common waveguides, are each directed to apredetermined set of delay lines, and subsequently separated andchanneled into N separate waveguides. The original incoming wavefrontscarried by the synthesized optical beams are now differentiated and canbe photodetected and analyzed separately.

This invention discloses novel ways to perform these functions utilizingphotonic beamforming networks. It provides methods for constructinglow-loss, completely passive, high performance photonic beamformingnetworks that can simultaneously control beam scanning and delay lineselection for multiple beams. The invention comprises three main methodswhich include:

(1) laser wavelength hierarchies,

(2) arrangements of wavelength division multiplexing (WDM) components,and

(3) re-use of laser wavelengths.

Multiple laser wavelengths are arranged in groups and subgroups(wavelength hierarchies) in the wavelength domain. By switching betweenthese wavelength groupings, the arrangements of WDM components proposedherein enable the beamforming network to direct the beam signals throughthe proper time delay lines, and to differentiate multiple beams. Noswitching occurs within the network itself, only at the controllinglasers, and the network is completely passive. Furthermore, signalrouting, beam differentiation, and beam vector summation occur withminimal losses due to the arrangements and choice of WDM components andinterconnections. The invention also minimizes the number ofphotodetectors required, and only one photodetector per beam is neededin its most powerful form.

The method of laser wavelength re-use permits significant reduction inthe number of wavelengths required for the beamformer to function. Thisallows the wavelengths to be limited to the standard ones specified bythe International Telecommunication Union, even with phased arrayantennas that contain a very large number of elements.

Another aspect of the invention, a non-passive, output-switched network,that minimizes the number of wavelengths required is also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b depict the two types of delay line networksconventionally used, Network Switched and In-Line Switched,respectively.

FIG. 2 is a schematic block diagram overview of an embodiment inaccordance with the present invention applied to a HeterodyneTrue-Time-Delay system.

FIG. 3 a is a table illustrating the wavelength hierarchy required forthe embodiment of FIG. 2

FIG. 3 b is an example of the laser wavelengths activated when beam 1 isdirected to port A and beam 2 is directed to port B.

FIG. 4 shows how a 1×3 WDM conceptually operates.

FIG. 5 is a block diagram depiction of box 1 from FIG. 2. It shows thespecific arrangement of WDMs and interconnections required to performthe functions discussed in conjunction with FIG. 2 and FIG. 3.

FIG. 6 is a variation of the network illustrated in FIG. 5, where WDMshave been replaced with fiber splitters.

FIG. 7 is a variation of the network illustrated in FIG. 5, where WDMshave been replaced with fiber combiners.

FIG. 8 is another variation of the invention of FIG. 5, utilizing portsplitters and electrical switches to reduce the number of wavelengthsrequired.

FIG. 9 is another variation of the invention of FIG. 5, where port andbeam demultiplexers have been replaced with port-beam demultiplexers.

FIG. 10 is another variation of the invention of FIG. 5; wherephotodetectors have been placed at the outputs of each fibermultiplexer, and the signals are electrically combined.

FIG. 11 is a table illustrating the wavelength hierarchy required inconjunction with the embodiment of FIG. 8.

FIGS. 12, 12 a and 12 b depict a three-dimensional structureconstituting a Network Switched delay line network for the case of a 4×4fiber array feeding four delay ports.

FIG. 13 is a block diagram of the embodiment of FIG. 5, where WDMs havebeen rearranged to place all cross-overs in one section of the network.

FIG. 14 shows how the 4×4 fiber array of FIGS. 12 a and 12 b can bedivided into two 2×4 arrays in conjunction with the wavelength re-usetechnique.

FIG. 15 a shows the WDM architecture to be employed with the wavelengthre-use method.

FIG. 15 b is a simplified block diagram of FIG. 15 a, and represents anetwork cell.

FIG. 16 shows how the network cells of FIG. 15 b would be connected inconjunction with the wavelength re-use method, for the case of a 4×4fiber array.

FIG. 17 is a table illustrating the wavelength hierarchy required inconjunction with the embodiment of FIGS. 18 and 19.

FIG. 18 is another variation of the invention where WDM filters are usedfor port and beam operations only. B×P×F photodetectors are required.

FIG. 19 is yet another variation of the invention where WDM filters areused for port and beam operations only as in FIG. 18, but requiring onlyB×F photodetectors because of the use of optical switches.

DETAILED DESCRIPTION

The present invention utilizes simple Wavelength Division Multiplexing(M) technology in a number of ways. FIG. 4 shows, for N=3, how a 1×N WDMoperates. A single common fiber 1 carries N wavelengths to a dispersiveelement 2 which spatially separates these wavelengths, and directs themto N single wavelength fibers 3. This device can be run in reverse as anN×1 combiner. In this mode, it is assumed that if the wrong wavelengthis in one of the single wavelength fibers, it does not couple into thecommon fiber. The wavelength bands passed by the WDM are adjustableduring fabrication.

Referring to FIG. 2, a schematic block diagram overview of an embodimentof the invention applied to a Heterodyne True-Time-Delay system isshown. This particular system, has the capability to receive B=2incoming wavefronts and direct each one to a separate output utilizingB=2 synthesized laser beams (beam1 and beam2), F=4 optical fibers (α, β,γ, δ) and P=3 ports or delay lines (A,B,C). For ease of understanding,it is further assumed that the system is set to direct beam 1 to port Aand beam 2 to port B.

Wavefronts 20 and 21 at respective frequencies f₀₁ and f₀₂ are receivedby antenna array 22.

Wavefronts 20 and 21 are detected and then travel down a set of feedlines 23.

Analog or digital beam-forming circuit 24 generates local oscillatorwavefront 26. Wavefront 26 is in the electrical domain at RF frequencyf_(LO1) and is defined by the relative phases of frequency f_(LO1) infour electrical cables or waveguides α_(LO1), β_(LO1), γ_(LO1), δ_(LO1).These electrical signals are used by port/beam selection block 28 tointensity modulate four lasers whose output intensities then bear thesame relative phases as the local oscillator wavefront 26. These fouroptical signals are then output to four optical fibers α₁, β₁, γ₁, δ₁where the relative phases of the light intensity among the fibers definean optical wavefront 26A. Symmetrically, analog or digital beam-formingcircuit 25 generates local oscillator wavefront 27. Port/beam-selectionblock 29 converts wavefront 27 to optical wavefront 27A and outputs, toeach of its four output fibers (α₂, β₂, γ₂, δ₂), a single wavelengthaccording to the specific delay line (port) desired for beam 2.

WDM 30 acting in combine mode, directs laser light from α₁ and α₂ intofiber α, laser light from β₁ and β₂ into fiber β, laser light from γ₁and γ₂ into fiber γ, and laser light from δ₁ and δ₂ into fiber δ. Thus,each optical fiber α, β, γ and δ, forming set of feed lines 33, carriestwo wavelengths. This results in two optical wavefronts, 31 and 32,traveling through set of fiber lines 33 at local oscillator frequenciesf_(LO1) and f_(LO2) respectively.

Incoming wavefront 20 and synthesized wavefront 32 intersect one anotherin mixers 34 and line by line mixing of the two wavefronts occurs. Suchmixing up-converts or down-converts the f₀₁ frequency to intermediatefrequency f_(IF1). Similarly, wavefront 21 and synthesized wavefront 31intersect one another in mixers 34 and line by line mixing of the twowavefronts produces intermediate frequency f_(IF2). Wavefronts 36 and 37travel down set of feed lines 35 and enter WDMs 38. Wavefronts 36 and 37are each directed to the desired port through the corresponding set ofdelay lines 39 (port A), 40 (port B) or 41 (port C). In the particularexample of FIG. 2, WDMs 38 outputs beam 1 to port A and beam 2 to portB. Wavefronts 42 and 43 travel through set of delay lines 39 and 40,respectively, and enter WDMs 44.

Within WDM 44, wavefront 42 (beam 1), entering through delay lines set39, is vector summed into a single fiber and directed to photodetectingdevice 46. Electrical signal 48 (corresponding to beam 1) is output byphotodetecting device 46 and is sent to a data processing unit.Similarly, wavefront 43, entering through delay lines set 40, is vectorsummed into a single fiber and directed to photodetecting device 45.Electrical signal 47 (corresponding to beam 2) is output byphotodetecting device 45 and is sent to a data processing unit.

Thus, incoming wavefronts 20 and 21 which were detected simultaneouslyhave been differentiated by the system forming the present invention,and the information they carry can be processed separately.

Although a specific configuration is treated in FIG. 2, the sameprinciples hold for, and the invention is applicable to, two-dimensionalsystems, for any values of B, F, and P, and to other types ofbeamforming devices such as the Heterodyning Rotman beamformer.

FIG. 3 a shows the WDM wavelength hierarchy required for the embodimentof the invention shown in FIG. 2. For such a system, it is necessarythat Nλ=24 (P×B×F=3×2×4) wavelengths be available to blocks 28 and 29 ofFIG. 2 (12 for block 28, and 12 for block 29). Block 28 will activateone of wavelength ranges A1, B1 or C1, depending on which port isselected for beam 1, block 29 will activate one of wavelength ranges A2,B2 or C2 according to which port is chosen for beam 2. Wavelength rangesA1, B1, C1, A2, B2, C2 are each composed of F=4 wavelengths asillustrated in FIG. 3 a. Thus, port and beam selection is accomplishedby switching groups of F=4 wavelengths for each beam. FIG. 3 b showswhich specific wavelengths need to be activated in order to direct beam1 to port A and beam 2 to port B. To achieve this, the F=4 wavelengthscorresponding to the A1 range, namely λ_(Aα1), λ_(Aβ1), λ_(Aγ1),λ_(Aδ1), and the F=4 wavelengths corresponding to the B1 range, namelyλ_(Bα2), λ_(Bβ2), λ_(Bγ2), λ_(Bδ2) are activated.

Expansion to include more fibers, beams, or ports is accomplished byadding additional wavelengths to either side of the range shown in FIG.3 b.

Referring to FIG. 5, the specific arrangement of WDM filters andinterconnections required to perform the functions discussed inconjunction with FIG. 2 and FIG. 3, is shown. FIG. 5 is a detailed blockdiagram representation of box 1 from FIG. 2. The light paths for beam 1going through port A (heavy solid lines) and beam 2 going through port B(heavy dashed lines) are highlighted in the figure.

Light in fibers α, β, γ, δ, enters 1×3 port demultiplexer WDMs 60. Theinput fiber to each of these WDMs carries two wavelengths (one for beam1 and one for beam 2 corresponding to wavefronts 36 and 37 of FIG. 2)and corresponds to common fiber 1 of FIG. 4. Each of the three outputfibers of each WDM transmits a single-wavelength range A, B, or C asdefined in FIG. 3 a. Thus, depending upon which wavelength group A1, B1,C1, A2, B2, C2 enters, the WDMs select A, B or C sets of delay lines forbeam 1 and beam 2. For example, let us consider fiber a and thecorresponding WDMα 86 (FIG. 5). Two wavelengths (one for beam 1 and onefor beam 2) traveling through fiber α enter WDMα. If both wavelengthsare in the A wavelength range, one in the A1 range and one in the A2range, WDMα will direct both wavelengths to delay line 61 (heavy solidline); If both wavelengths are in the B wavelength range, one in the B1range and one in the B2 range, WDMα will direct both wavelengths todelay line 62 (heavy dashed line); If both wavelengths are in the Cwavelength range, one in the C1 range and one in the C2 range, WDMα willdirect both wavelengths to delay line 63 (light solid line). If onewavelength, say λ_(Aα), is in the A range (A1 or A2) and the otherwavelength, say λ_(Bα), is in the B range (B1 or B2), then WDMα willdirect λ_(Aα) to delay line 61 and λ_(Bα) to delay line 62. If onewavelength, say λ_(Aα), is in the A range (A1 or A2) and the otherwavelength, say λ_(Cα), is in the C range (C1 or C2), then WDMα willdirect λ_(Aα) to delay line 61 and λ_(Cα) to delay line 63. If onewavelength, say λ_(Bα), is in the B range (B1 or B2) and the otherwavelength, say λ_(Cα), is in the C range (C1 or C2), then WDMα willdirect λ_(Bα) to delay line 62 and λ_(Cα) to delay line 63. Note thatalthough WDMα has three output fibers, a maximum of two are active atany given time since only two wavelengths enter the WDM.

After passage through delay line set 61, 62, or 63, light from the fourfibers of each delay line set next encounters beam demultiplexer WDMgroups 64, 65 or 66. Each of these groups comprises four 1×2 WDMs.Wavelength ranges A1 and/or A2 enter WDM group 64 and get separated.Wavelengths in the A1 range are directed to fiber multiplexer WDM 67,and wavelengths in the A2 range are directed to fiber multiplexer WDM68. In the same fashion, wavelength ranges B1 and/or B2 enter WDM group65 to be separated. Wavelengths in the B1 range are directed to fibermultiplexer WDM 69, and wavelengths in the B2 range are directed tofiber multiplexer WDM 70. Lastly, wavelength ranges C1 and/or C2 enterWDM group 66 and get separated. Wavelengths in the C1 range are directedto fiber multiplexer WDM 71, and wavelengths in the C2 range aredirected to fiber multiplexer WDM 72. This operation serves to place thebeam 1 light on one fiber and the beam 2 light on the other fiber at theoutput of each 1×2 WDM comprised in WDM groups 64, 65 and 66.

Each fiber multiplexer WDM 67, 68, 69, 70, 71, and 72 receives lightfrom four input fibers and combines them into a single output fiber. Thefour input fibers of each fiber multiplexer, each carry the individualwavelengths α, β, γ, δ shown under the A1, B1, C1, A2, B2, C2 ranges inFIG. 3 a. Combination of the light from the four input fibers by thefiber multiplexers serves to vector sum the envelopes of the of theoptical carriers and form the beams. Fiber multiplexers WDMs 67, 69 and71 direct their single outputs to beam 1 multiplexer 73, through fibers78, 79 and 80 respectively. Fiber multiplexers WDMs 68, 70 and 72 directtheir single outputs to beam 2 multiplexer 74, through fibers 81, 82, 83respectively. Fibers 78, 79 and 80, are then merged into a single fiber84 by beam 1 multiplexer 73, and fibers 81, 82, 83 are merged into asingle fiber 85 by beam 2 multiplexer 74. The input passbands of beam 1multiplexer 73 are wavelength ranges A1, B1, and C1. The input passbandsof beam 2 multiplexer 74 are wavelength ranges A2, B2, and C2. Beam 1,traveling through fiber 84, is photodetected by photodetecting device75, and beam 2, traveling through fiber 85, is photodetected byphotodetecting device 76. Only one photodetecting device per beam isrequired. Beam 1 always appears at the beam 1 output port and beam 2 atthe beam 2 output port, independent of the beam scan angle and whichdelay line sets were chosen.

In an alternative embodiment of the basic invention of FIG. 5, the portdemultiplexers 60 and beam demultiplexers 64, 65, 66 are replaced withsimple fiber splitters as illustrated by FIG. 6. This is possiblebecause the filtering performed by the port and beam demultiplexers 60,64, 65, 66 is redundant to the filtering performed by the fibermultiplexers 67, 68, 69, 70, 71, and 72. Referring to FIG. 6, the portand beam demultiplexers of FIG. 5, have been replaced by four 1×6 fibersplitters. The rest of the configuration remains the same, and the samefunctions are performed. Alternatively, the fiber and beam multiplexerscan be replaced with combiners without affecting the performance of thenetwork. This configuration is shown in FIG. 7, two 12×1 combiners 100and 101 replace fiber and beam multiplexers 67, 68, 69, 70, 71, 72, 73,and 74 of FIG. 5.

Referring to FIG. 8, yet another embodiment of the invention isillustrated. In this variation, the port demultiplexers 60 of FIG. 5 arereplaced with the same number of 1×3 fiber splitters 110. Photodetectiontakes place at the output of fiber multiplexers 113 and is performed byphotodetectors 111. Beam multiplexers 73 and 74 of FIG. 5 are replacedwith electrical switches 112, which permit to select the delay linedesired for each beam. The addition of electrical switches eliminatesthe need for the laser wavelengths used to select a delay line set. Thusthe configuration of FIG. 8 reduces the total number of wavelengthsrequired from Nλ=P×B×F=24 to Nλ=B×F=8. If the beamformer has more thanP=6 ports, the present configuration will have higher losses than thepure WDM configuration of FIG. 5, but will require P times fewerwavelengths. FIG. 11 shows the wavelength hierarchy required for theconfiguration of FIG. 8. These wavelengths are used only for beamseparation and incoherent summation.

The wavelength hierarchy of FIG. 11 is a truncated version of the oneshown in FIG. 3 b, where the empty wavelength slots of FIG. 3 b areeliminated by the use of electrical switches instead of wavelengthranges for port selection.

In another variation of the invention, the port and beam demultiplexersof FIG. 5 can be replaced with a single port/beam demultiplexer. In thisconfiguration the delay lines cannot be shared by the beams, and B timesas many delay lines are needed. On the other hand, the use of a 1×Z WDM,instead of a 1×X and a 1×Y WDM can reduce losses for X<6 and Y<6 butZ=X×Y>6. In the particular example where P=3, B=2, and F=4, 1×3 port and1×2 beam demultiplexers 60, 64, 65, and 66 of FIG. 5, are replaced with1×6 port/beam demultiplexers, as illustrated by FIG. 9. These 1×6port/beam demultiplexers would have six output fibers with passbands A1,A2, B1, B2, C1, C2, as shown in FIG. 3A. The output fibers are connectedto corresponding fiber multiplexers 67, 68, 69, 70, 71 and 72 of FIG. 5.

Another useful variation of the basic invention presented in FIG. 5, isto place a photodetector at the output of each fiber multiplexer 67, 68,69, 70, 71 and 72, as shown in FIG. 10. Then, the outputs of thephotodetectors placed after fiber multiplexers 67, 69, and 71 (A1, B1,C1 respectively) can be electrically combined into beam 1, and theoutputs of photodetectors placed after fiber multiplexers 68, 70, and 72(A2, B2, C2 respectively) can be electrically combined into beam 2. Thisis possible because only one of the three outputs from A1, B1, C1 isactive at any given time, and only one of the three outputs from A2, B2,C2 is active at any given time. This configuration does not require anyswitching and is completely passive. It has an important application inwavelength re-use networks and is discussed below.

Generally, phased array antennas operate in two dimensions and requiretwo-dimensional delay line networks. FIG. 12 shows the case of a 4×4fiber array 120 feeding P=4 delay ports A, B, C, and D. A total ofF×P=16×4=64 delay lines are required. For simplicity, FIG. 12 a showsonly delay lines 121, 122, 123 and 124, connecting fiber γ of the toprow of array 120, to the four ports A, B, C, and D. For clarity, thedelay lines that feed port C only are illustrated in FIG. 12 b. Each ofthe F×P delay lines that constitute the system has a path length welldefined that is determined by the system geometry, and the velocity oflight in the delay lines. Equations 125 show the path length differencesin the x and y directions, ΔLx and ΔLy, from one fiber to the next whengiven the geometry of the system (i.e., antenna element spacing D, anddelay line scan angle components θx and θy) and the velocity of light inthe delay line.

If implemented using fibers for the delay lines, the delay line networkof FIG. 12 would be difficult to fabricate, and would require carefulcutting to a specific length and splicing F×P fibers. However, thisthree-dimensional structure can be collapsed into two dimensions andfabricated along with the WDMs in an integrated structure using silicaon silicon (SOS) waveguide technology. This collapse to two dimensionsis possible because there is a unique mapping of length between inputfiber and output port. For example, referring to FIG. 12 a, fiber 121(fiber γ of top row) connected to port A, has a length that is unique inthe network. This is true for all of the F×P fibers that constitute thenetwork. Consequently, each fiber can be placed on a flat surface. Aslong as their respective lengths are respected, this new two-dimensionalstructure is equivalent to the three-dimensional architecture of FIG.12, and can perform the same functions. It is to be noted that whencollapsed into two dimensions, the delay line waveguides will cross overeach other, and slightly increase network loss. While low-losscross-overs are easily made using SOS, minimal loss in the network isachieved by minimizing the number of cross-overs. In FIG. 5, cross-oversoccur in all three areas between the four columns of WDMs. Tryingdifferent arrangements of the WDMs within each column, while keeping theinterconnections the same, indicates that placing cross-overs inmultiple areas minimizes their number. The network of FIG. 5 yields theminimum number of cross-overs for the arrangements tried. It is thus agood candidate for a low-loss structure made with SOS. WDMs of the ArrayWaveguide Grating (AWG) type, can be fabricated using SOS, and easilyintegrated on the same substrate as the crossing waveguides and/or delaylines. The arrangement of FIG. 5 lends itself to standardization andfabrication of the network on one or on multiple substrates. Forexample, a standard set of interconnections and routing WDMs 132 (FIG.5) could be made on one substrate, and a standard input interface 130could be made on another. Application specific delay lines 131 couldthen be made and incorporated into the network. This approach wouldsubstantially lower the cost of making large numbers of photonicbeamformers that only differ with respect to the delay lines required.Special attention to reducing losses could be paid, since theinterconnection and routing section 132 which has the most cross-overs,would be one of the standard pieces.

Referring to FIG. 13, an example of how the WDMs can be rearranged onthe substrate placing all cross-overs in one section, is shown. Thisarrangement has more cross-overs and thus higher losses than thearrangement of FIG. 5. While the network of FIG. 5 was developedassuming a network-switched architecture, the rearranged system of FIG.13 looks like an in-line switched network (compare with FIG. 1). The WDMsystem can be thus made to look like either network type simply bymoving the WDMs and waveguides around while keeping the interconnectionsthe same. Therefore, this invention may be used equally well innetwork-switched and in-line switched architectures.

If standard ITU wavelengths in the 1550 nm band are used with a spacingof 50 GHz, the system is limited to approximately 100 wavelengths. Thisnumber can be doubled using the non-standard 1300 nm band to a maximumof Nλ=P×B×F=200 wavelengths. Most practical systems have around 100fibers, leaving little room for beam and port operations. To overcomethis limitation, methods of wavelength re-use will now be disclosed.

The WDM techniques described herein are well suited to wavelengthre-use. The general approach, as illustrated in FIG. 14, is to dividethe two-dimensional array 134 of input fibers into M sub-arrays or cells135 and 136, in such way that each cell uses W=P×B×(F/M) wavelengths.Thus, for M≧2 W is a fraction of the number of wavelengths needed in theoriginal network. Each cell uses the same set of W wavelengths, hencethe term “wavelength re-use”. The outputs of each cell, after goingthrough the delay lines/ports (A, B, or C in FIG. 14), are separatelyphotodetected. After photodetection the RF outputs of the photodetectorsare electrically summed with equal length or corporate feeds to form theoutput beam. The cells can be formed of any subset of the fiber array,rows, columns, parts of rows or columns, or even randomly chosenelements throughout the array. All that is required is that the inserteddelays be proper. The beamforming network for each cell is independentand self-contained. FIG. 14 illustrates the particular example of a 4×4array of fibers divided into M=2 cells, each cell including 2×4 fibers.Each cell feeds P=3 ports, with only one beam being assumed in thisexample for simplicity. In general, the cells have different sets ofdelay lines because they represent different parts of the Rotman lens.However they all use the same wavelengths, WDM structure, andinterconnect architecture. Consequently, this configuration lends itselfwell to the cost-saving standardization earlier described. Indeed, allcell networks preferably use identical input interfaces andinterconnect/routing modules, and differ only in the amount of delayinserted by the delay lines utilized.

FIG. 15 a shows the WDM architecture utilized in conjunction with thewavelength re-use method. Note that this structure is identical to theone shown in FIG. 5, except for the beam multiplexers which have beenremoved and replaced by photodetectors disposed at the outputs of eachfiber multiplexer. After photodetection the signals can be electricallycombined as discussed previously. FIG. 15 b is a simplified blockrepresentation of the FIG. 15 a. Since wavelength re-use configurationutilizes M times less wavelengths as the embodiment of FIG. 5, it willutilize M cells 140 of the type shown in FIG. 15 b, to accomplish thesame functions. The first cell uses a set of W wavelengths, and allsubsequent cells “re-use” the same set of W wavelengths.

FIG. 16 shows an example of how cells 140 can be connected to performthe functions of a 4×4 fiber array, in accordance with the wavelengthre-use method. The system of FIG. 16 handles B=2 beams and P=3 ports,and is a two dimensional system, contrary to prior art fiber arrayswhich are three-dimensional. Cells 151, 152, 153, and 154 represent rows1, 2, 3, and 4 of the 4×4 fiber array, respectively. Each cell is anindependent WDM network with P×B electrical outputs A1, B1, C1, A2, B2,C2, and each require the same P×B×(F/M)=3×2×(16/4)=24 wavelengths.

The cells are identical except for the length of the delay lines. Theoutputs A1, A2,B1, B2, C1, C2 of each cell are directed to thecorresponding beam summation junction 155, 156, 157, 158, 159, 160,respectively, via corporate feed 161 (i.e., all A1 cell outputs aredirected to A1 summation junction 155, all A2 cell outputs are directedto A2 summation junction 156, and so on). For clarity, only three feedlines corresponding to A1, A2, and C2 are shown in FIG. 16. The foursignals entering A1 beam summation junction 155 are then vector summedinto a single beam, and the same occurs at beam summation junctions 156,157, 158, 159 and 160. The A1, B1, C1 junctions, 155, 156, 157,respectively, are then fed to a common beam 1 output 162. The A2, B2, C2junctions, 158, 159, 160 respectively, are then fed to a common beam 2output 163. The network requires no switching and is completely passive.

A fiber array with F fibers and M cells will require B×P×Mphotodetectors. The number of photodetectors needed is independent ofthe number of fibers in the array, and is a significant reduction overmore conventional networks that do not use a different wavelength foreach fiber involved in the vector summation.

For example, if WDMs were used just for port and beam operations in anetwork-switched architecture using the wavelength hierarchy of FIG. 17,a photodetector would have to be placed on every fiber as shown in FIG.18. This would require either B×P×F detectors as shown in FIG. 18, orport switching of B detector arrays each containing F detectors (for atotal of B×F detectors) as shown in FIG. 19. The number ofphotodetectors needed increases while the number of wavelengths requireddecreases. For the example of FIG. 18, 24 photodetectors are neededwhile only 6 wavelengths are required for the system to function. SinceM can be much less than F, if one re-uses a large number of wavelengths,B×P×M can be much less than both B×P×F and B×F, and thus the number ofphotodetectors required can be largely reduced. A trade-off, between thecost and complexity of adding laser wavelengths versus reducing thenumber of photodetectors, must be made for each particular photonicbeamforming system.

Fiber splitters may be substituted for WDMs as discussed previously.Furthermore, if switching is used at the output of the A1, A2, B1, B2,C1, C2 junctions, a beam-fiber wavelength hierarchy similar to FIG. 11can be used to further reduce the number of wavelengths required toB×(F/M)=8. In this case, B=2 switches, each having P=3 possiblepositions, are required.

Having described the invention in conjunction with certain embodimentsthereof, modifications and variations will now certainly suggestthemselves to those skilled in the art. As such, the invention is notlimited to the disclosed embodiments except as required by the appendedclaims.

1. A communication method comprising: providing an antenna array tosimultaneously receive incoming RF wavefronts; amplitude modulating alaser light to provide synthesized optical beams; mixing the incoming RFwavefronts with the synthesized optical beams to form opticalwavefronts; directing said optical wavefronts to a predetermined set ofdelay lines; and separating and channeling said optical wavefronts intoseparate waveguides.
 2. The method of claim 1, wherein mixing of theincoming RF wavefronts with the synthesized optical beams is performedby optical modulation.
 3. The method of claim 1, further comprisingseparately photodetecting and analyzing the separated optical beams.